For realism should I use the maximum number of Subsurf levels?
My advice would be the exact opposite:

Use the minimum amount of subdivision that will give you acceptable results.

How much that is depends on the complexity of the shape and the resolution of the images you want to generate.

Needlessly increasing the amount of subdivisions comes at a price: RAM (memory) usage and computer processing. A higher number of vertices will impact your computer's performance. Keep in mind that for every level of subdivision you are increasing the number of vertices exponentially, so it's quite easy to end up with geometry that exceeds your computing capacity.

For more details read Render crash using subsurf modifiers.

3d realism requires taking clever shortcuts and fooling the viewer. Use high level of detail only on those areas that are featured prominently on the final image(s), objects that are going to be far away need less detail than those up close.